A conventional thermal fixing device used in an electrophotographic image forming apparatus includes an endless fixing belt (tubular member), a heater disposed at an internal space of the tubular member, a backup member and a heating plate (nip member) that nips the tubular member in cooperation with the backup member. In this fixing device, a recording sheet is conveyed between the tubular member (the nip member) and the backup member for thermally fixing a developer image to the recording sheet.